The invention relates to electronic devices, and, more particularly, to integrated circuit accelerometers and methods of fabrication.
Accelerometers detect mechanical acceleration and typically provide an electrical output signal which depends upon the amplitude of the acceleration. Applications for accelerometers include crash detection for airbag activation in automobiles. Incorporating an accelerometer on an integrated circuit, such as illustrated in FIG. 1, permits signal conditioning circuitry and signal processing circuitry to be included with the accelerometer and potential low cost fabrication. Thus analysis of detected accelerations may be made on the same integrated circuit as the accelerometer itself. In an automobile crash the high acceleration should persist for a few milliseconds, so there is sufficient time for electronic analysis of the acceleration and consequent control signals for various actions, including deployment of an airbag. Analog Devices Inc. produced the ADXL50 integrated circuit accelerometer with capacitive detection of motion of an array of polysilicon fingers affixed to the surface of a silicon integrated circuit. This accelerometer was designed to detect accelerations up to .+-.50 g in one direction parallel to plane of the integrated circuit. See Electron Design, Aug. 8, 1991, pages 45-56. However, the inertial mass of the ADXL50 is not large and detection of smaller amplitude acceleration, such as .+-.1 g, would require an enormous array and not be practicable.
U.S. Pat. No. 5,192,395 discloses an integrated circuit accelerometer including an array of deflectable elements which are squares (50 .mu.m long side) of an aluminum alloy and suspended about 2 .mu.m over a contact by hinges connected to a ground plane. An acceleration which deflects a suspended square to the contact in effect closes a switch and this is electrically detected. The various deflectable elements have either differing masses or hinges with differing restoring constants. Thus a measurable acceleration will cause the elements with large masses or low restoring constants to deflect to their contacts but not the elements with small masses or high restoring constants. Thus detecting the transition from deflected-to-contact elements to not-sufficiently-deflected elements in the array gives a measure of the amplitude of the acceleration.
However, the known integrated circuit accelerometers have the problem of large size required for detection of small amplitude accelerations.
U.S. Pat. No. 4,954,789 discloses a spatial light modulator with an array of essentially square deflectable elements suspended by hinges connected to posts about the corners.
Electroless plating of nickel from either acidic or basic solutions of phosphorus or boron compounds as the reducing agent is known. In fact, electroless plating of nickel on roughened silicon for micromechanical structures such as interdigitated electrodes for a lateral resonant microactuator appears in Furukawa et al, Electroless Plating of Metals for Micromechanical Structures, Proc. Transducers (June 1993, Yokohama).